1. Field of the Invention
This invention relates to an acoustic attenuator for attenuating acoustic waves traveling along a well tool and to a method of using such a tool to attenuate acoustic waves.
2. Background of the Art
In hydrocarbon exploration, acoustic logs are commonly run to obtain the speed of propagation of sound in the formations surrounding a borehole. Of particular interest is obtaining acoustic logs during the process of drilling, i.e., "measurements-while-drilling", known as MWD measurements. MWD measurements are now routinely obtained of neutron scattering, gamma ray scattering and electrical resistivity of underground formations. However, obtaining MWD acoustic logs has not been as successful due to the poor signal to noise ratio of acoustic log signals.
Acoustic logs are obtained by using a transmitter to generate an acoustic signal in the borehole and using a receiver at some distance from the transmitter to measure a received signal. Both the transmitter and the receiver are located on a drilling collar in proximity to the drilling tool. The borehole is filled with a drilling mud to facilitate the drilling process. The received signal consists of many components, the one of most interest being a component that travels through the borehole fluid into the formation, then as an acoustic wave in the formation to a point near the receiver from whence it travels as to the receiver in the borehole fluid. In addition to this desired signal, the transmitter itself excites a number of other types of signals that are received by the receiver, including borehole waves, tube waves and direct signals through the tool body.
The term acoustic as used herein is intended to include all types of elastic waves, including sound waves through fluids and compressional and shear waves in solids. The velocity of sound through fluids used in well bores is approximately 5,000 feet per second. On the other hand, the velocity of compressional waves through earth formations typically ranges from 4,500 ft/s (for porous sandstones with low gas saturation) to about 22,000 ft/s for nonporous carbonate rocks. By way of comparison, metals can have compressional wave velocities ranging between 13,000 and 20,000 feet per second. The velocity of borehole waves (tube waves, Stoneley waves, etc.) is somewhat less than the velocity of compressional waves in the formation.
Since the speed of propagation of compressional waves in the tool body, which is normally steel, is commonly much higher than that of the formation rock, the tool signal arrival usually occurs before the formation arrival. As an acoustic tool merely records signals as they are obtained, the tool has no way of distinguishing whether a signal has traversed the formation or the tool body. Thus, such a first arriving signal propagated along the tool body may be confused as the first arriving signal traversing the formation. In addition, in an MWD environment, the drillbit itself acts as a signal source that sends out signals that may be much stronger than the signal generated by the transmitter in the acoustic tool: the transmitter signal maybe swamped by the drillbit signal.
In wireline applications, the problems caused be signals originating at the drilibit are not present. Additionally, the tool is not required to function as a load bearing member so that it has been possible to form an array of staggered openings through the width of the sidewall of the tool's housing. These openings serve to lengthen the total path length that an acoustic signal propagated through the housing must traverse so that the signal across an extremely broad range of frequencies is not only delayed in its transit of the array of holes, but is also attenuated as a result of the increased path length and the signal scattering caused by the openings.
In MWD applications, making cuts that extend through the side wall thickness of the acoustic well tool is clearly unsatisfactory because an acoustic tool that is incorporated into the drill collar and must be able to withstand the immense forces and accelerations encountered during the drilling of the well. A large numbers of perforations through the side wall of the drill collar would weaken the collar so that it would no longer be able to withstand normal wear and tear of drilling. In addition, in MWD applications, drilling fluid is conveyed from the surface to the drillbit under pressure on the inside of the drill collar and the returning fluid from the drillbit to the surface travels on the outside of the drill collar. The return fluid carries with it cuttings from the bottom of the hole. Making holes through the drill collar means that an additional internal tubing would be required to carry the drilling fluid down from the surface.
U.S. Pat. No. 5,510,582 discloses a device in which an acoustic attenuation section is positioned between the transmitter and the receiver of the acoustic well tool. This acoustical attenuation section generally includes one or more cavities in the acoustic well tool, into which are inserted inertial masses. The cavities are generally shaped to receive the inertial masses and are slightly larger so that a gap will exists between the walls of the cavities and the inertial masses as the inertial mass is positioned in the cavity. Residing in the gap is an attenuation fluid. An o-ring seal keeps the fluid within the cavity. A locking cap member serves to keep the inertial mass positioned within the cavity. As noted in the patent, the results showed that merely cutting slots or firmly securing fillers in the slots gave smaller attenuation than using slots with inertial masses surrounded by an attenuation fluid in a gap of proper thickness for the frequency and fluid viscosity. The arrangement disclosed in U.S. Pat. No. 5,510,582 is rather complicated, making it difficult to machine. In addition, close tolerances in the machining are required in order to maintain the fluid gap between the inertial masses and the walls of the cavity. A supply of the inertial masses needs to be kept available, and changing the inertial masses would require considerable time, including possibly down time of a rig.
Gas bubbles in fluids are known to cause a significant attenuation of ultrasound. As the sound wave passes, the acoustic energy is dissipated in excited resonance bubbles and is thus lost. The sound wave continues to propagate with reduced energy while the bubbles continue to oscillate until viscous damping stops the oscillation. There are several conditions for such a resonance bubble. The compressibility of the gas bubble must be much greater than the compressibility of the surrounding fluid. The wavelengths of the ultrasound must be much larger than the diameter of the bubble. The surrounding fluid must deform in response to deformation in the gas bubble.
In principle, liquid bubbles in solid steel could also be used to dampen waves propagating in the steel as the conditions noted above are met. Such a device would be mechanically robust and simple. One embodiment of the present invention is based upon this principle.